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A Guangzhou Honda accord 2. 3L car with a mileage of about 185 thousand km and an engine type of F23A3. Due to a traffic accident, the engine room was damaged to a large extent and entered the factory for maintenance. For shaping and painting operations, the engine is removed as a whole. After the accident was partially repaired, the engine was tested, but the engine failed to start. Fault diagnosis: the maintenance technician after the inspection found no high pressure fire, so the replacement of the distributor assembly (inside the cylinder to identify the CYP sensor) and ignition coil, but the test found that the engine can not start. The fault codes obtained by the self diagnosis method of the electric control system are 4 and 8, which indicate the position sensor of the crankshaft position and the sensor of the top dead center or its line fault. The 2 sensors are installed behind the crankshaft pulley. The resistance of the sensor coil is measured, the result is two, so the upper stop point position sensor and the crankshaft position sensor are replaced, but the engine is still unable to take the machine. The spark plug test is still done without spark, and the injector is not working. The fuel pump doesn't work either. Once the fault code is cleared, the fault code is again retrieved. SCS short connection short connection repair diagnostic connector (located in the car driver's driver's side dashboard), connect the ignition switch, the result of the fault code is 4 and 8, and the same as the original. Check engine ECU power and grounding conditions, did not find abnormal conditions. For no electric spark, the injector does not move, the fuel pump does not work and other failure phenomena, how to judge the engine crankshaft position sensor signal should not be lost to the engine ECU. However, the top stop position sensor and the crankshaft position sensor have been replaced, and the connecting wire between the engine ECU and the 2 sensors has been checked. There is no problem, so it is only possible that the engine ECU itself has a problem. So I changed to a good ECU of the same type car, and when I tried to start it, the trouble remained. Description is not caused by engine failure due to ECU failure. At this point, the maintenance personnel really had no other way, so they decided to turn to the experts. Experts suggest that the output signal waveform of the top dead center position sensor and the crankshaft position sensor can be checked with an oscilloscope. If the oscilloscope does not check the output signal waveform of the sensor, it is necessary to check the installation of the 2 sensors, whether the position of the crankshaft is shifted or whether the positive time belt moves backward from the center of the pulley. According to the above indication, the oscilloscope is beaten to the normal voltage scale, and the signal waveform of the top dead center position sensor and the crankshaft position sensor is checked. Using the starter to drive the engine running, we can not see the output signal waveform of the sensor. Then the voltage scale of the oscilloscope is amplified and retried. The signal waveform on the oscilloscope shows that the maximum voltage of the sensor signal is only about 0.2 V, indicating that the signal voltage of the upper stop point position sensor and the crankshaft position sensor is too low, so that the engine ECU can not identify the rotating signal of the generator and the piston position signal. . Because the sensor is new, there should be no problem, but the resistance of the sensor is measured in normal range. Check the installation status of the 2 sensors, and there is no loosening. Then it feels that the gas gap of the sensor may be too large. According to the maintenance experience, the usual air gap of the crankshaft position sensor should be 0. 2mm to 0. 5mm, otherwise it is difficult to cause the change of the magnetic line of force, and of course, the signal voltage will not be output. To replace the crankshaft position sensor and the top stop position sensor, the most basic operation is to remove the crankshaft belt wheel, but for the installation of the sensor, only the fixed bolt can be tightened. When the engine rotates, it changes the magnetic force line instead of depending on the position of the mobile sensor to change its air gap. Since the position of the sensor can not be moved, it can only be considered that the position of the crankshaft pulley is shifted or the timing belt moves backward from the center of the pulley. The crankshaft pulley can be moved gently by striking the crankshaft pulley with a hand hammer. The fixed bolts of the crankshaft belt wheel were not tightened. The crankshaft pulley is gently knocked back by a rubber hammer, and the fixed bolt of the crankshaft is tightened again according to the standard tightening torque, and the engine test is started. The engine is immediately installed, and the above fault is excluded. Fault maintenance summary: the vehicle damaged the crankshaft position sensor when it collided or dismantled the engine. After replacing the sensor, the fault should have been eliminated. Unfortunately, it did not install it in place when the crankshaft was assembled and tightened the bolt according to the specified tightening torque. Adjust the timing position and start the engine at later time. The crankshaft wheel offset to the outside, at this time the air gap between the magnetic pole and the ring of the sensor is far beyond the normal value. The signal produced is weak, resulting in the ECU no ignition signal output, the spark plug of the engine, the injector no action, and the engine cannot start.